A gradient in the duration of the G1 phase in the murine neocortical proliferative epithelium

Neuronogenesis in the neocortical pseudostratified ventricular epithelium(PVE) is initiated rostrolaterally and progresses caudo- medially asdevelopment progresses. Here we have measured the cytokinetic parametersand the fractional neuronal output parameter, Q, of laterally locatedearly-maturing regions over the principal embryonic days (E12-E15) ofneocortical neuronogenesis in the mouse. These measures are compared withones previously made of a medial, late- maturing portion of the PVE.Laterally, as medially, the duration of the neuronogenetic interval is 6days and comprises 11 integer cell cycles. Also, in both lateral and medialareas the length of G1 phase (TG1) increases nearly 4-fold and is the onlycell cycle parameter to change. Q progresses essentially identicallylaterally and medially with respect to the succession of integer cellcycles. Most importantly, from E12 to E13 there is a steeply declininglateral to medial gradient in TG1. The gradient is due both to the lateralto medial graded stage of neuronogenesis and to the stepwise increase inTG1 with each integer cycle during the neuronogenetic interval. To ourknowledge this gradient in TG1 of the cerebral PVE is the first cellbiological gradient to be demonstrated experimentally in such an extensiveproliferative epithelial sheet. We suggest that this gradient in TG1 is thecellular mechanism for positionally encoding a protomap of the neocortexwithin the PVE.